This invention relates to a method for the mass production of squirrel-cage rotors for electric motors.
It is well known to produce squirrel-cage rotors by stamping a plurality of generally circular, high magnetic permeable laminations from thin steel sheet stock. The laminations each include a central bore and a plurality of identical generally radial notches circumferentially spaced at equal angular intervals about the outer margin of the lamination. The laminations are then stacked and compressed within a die casting mold to form a core having a longitudinal central bore therethrough and circumferentially spaced slots which extend longitudinally through the core at the outer margin thereof. The laminations are skewed such that the slots are wrapped slightly around the longitudinal axis of the core in a somewhat helical fashion. Molten metal is then injected into the slots formed by the laminations to produce spaced bars along the outer margins of the core as well as end rings which hold the laminations in place.
It is also known that in order to produce the very best motor performance possible, the conductivity of the bars should be as high as possible. It has been generally accepted that the bars should be formed from the highest purity aluminum and thus the highest conductivity aluminum, which is available. The aluminum which has been generally utilized by motor manufacturers has a very low iron content of about 0.1% to 0.2%. For purposes of maintaining the highest conductivity possible of the aluminum, it has been the desire of rotor manufacturers to obtain aluminum of even greater purity and having an even less iron content. The conventional thinking in the industry has been that any further contamination of the rotor aluminum with iron would degrade the performance of the rotor, thus producing an inferior motor.
A problem that has plagued mass producers of electric motors for many years is that when aluminum is injected into the rotor core to form the rotor, the aluminum inconsistently forms voids during the casting process. These voids are not detectable by visual inspection. The voids, however, exhibit themselves in electrical tests when the motors demonstrate overall poor performance and excessively noisy operation. The only practical ways for determining when voids are formed is to test each motor prior to shipment, or to have motors fail in applicational use.
It is known in the art that molten aluminum is very aggressive toward unprotected steels. That is to say, molten aluminum often solders to unprotected steels.
It was also common in past motor manufacturing procedures to heat treat laminations after punching to mitigate aluminum soldering in rotor casting. That is, stator and rotor laminations often still are heat treated to form an oxide layer on the bare metal. When oxidation steps are provided in a motor construction, they adds cost to the product and the degree of oxidation is hard to control. Consequently, even where oxidation steps are included in the motor manufacturing process, it still is possible to have production problems with rotors using conventional construction techniques.
I have found that contrary to the conventional thinking in motor manufacturing, rotor grade aluminum, that is, high purity aluminum exhibiting superior electrical performance per se, not only need not be used in rotor manufacture, but that overall motor performance can be improved when rotors are constructed according to the method of my invention.